The typical personal computing system employs a central processing unit, a video controller, and a video display device. The central processing unit provides address, data, and clock information to the video controller which interacts with the system memory and ultimately controls the images displayed in the video display device.
Traditionally, personal computing systems have used cathode ray tube (CRT) type display devices. More recently, however, many manufacturers and vendors have employed flat panel display devices. Among other advantages, a flat panel display can be fabricated thinner and more compactly than can a CRT display.
On the other hand, a flat panel display has a slower response time than a CRT display because it is a chemically operative system. This difference in response time requires differences in timing, sync, horizontal, vertical, and other display control functions for the two types of devices. These control differences in turn necessitate different hardware and circuitry in the controllers used with a flat panel display as compared to a controller used with a CRT. Presently, most commercially available processors and graphics controllers are designed to interface with only a CRT, or possibly a flat panel display, but not both.
In order to keep down costs and proliferation of models, manufacturers of processing systems and controllers prefer not to design a different processor and video controller merely because a flat panel display will be used in a processing system rather than a CRT. Thus, there exists a need for a commercially viable circuit and method for controlling the display for both a CRT and flat panel display device in a processing system.
Prior art patents known to applicants neither teach or suggest a solution to the foregoing problems. U.S. Pat. No. 4,338,597 pertains to a method for communicating between a CRT and its controller over long distances. U.S. Pat. No. 4,739,313 describes a method and circuit for using a composite video input to generate a plurality of video outputs. U.S. Pat. No. 4,563,676 discloses a circuit for generating a composite video signal. U.S. Pat. No. 4,626,837 describes a technique for superimposing video information. British Patent No. 2085257 involves a text mode display. The size of display characters is controlled by changing the frequency of the clock while the character is being displayed (horizontally) and by repeating the character line (vertically).